This invention relates to communication between a host computer and peripherals and more particularly to connect and disconnect signaling of devices on the Universal Serial Bus (USB).
The Universal Serial Bus (USB) is a peripheral bus specification developed by personal computer (PC) and telecommunication industry companies that brings the plug and play features of computer peripherals outside the PC box. This eliminates the need to install cards into dedicated computer slots and reconfigure the system. Personal computers equipped with USB allow computer peripherals to be automatically configured as soon as they are physically attached. This eliminates the need to reboot or run setup software. USB also allows multiple devices to run simultaneously on a computer, with dedicated hubs and peripherals such as monitors and keyboards also acting as additional plug-in sites.
The Universal Serial Bus is defined in the Universal Serial Bus Specification, revision 1.1, Sep. 23, 1998 which is hereby incorporated herein by reference. This document is available from the USB Implementers Forum web page at www.usb.org.
One aspect of the USB specification provides for the detection, by a host or hub device, of the connection and disconnection of downstream devices. This is done by the use of a 1.5Kxcexa9 pull-up resister between a positive supply voltage and one of the data lines (D+ or Dxe2x88x92). Since the host or hub device has its D+ and Dxe2x88x92 lines terminated to ground through a 15Kxcexa9 resistor, when no downstream device is connected these lines are read as both being a logical zero. When the downstream device is connected, the 1.5Kxcexa9 pull-up resistor causes one of the data lines to be pulled to a logical one. This informs the host or hub device that a new device has been connected. The host then queries the device for more information and may reset and configure the device shortly thereafter.
The USB specification also describes two types of devices: bus-powered and self-powered. Bus-powered devices rely totally on the USB cable for power. In contrast, those that have an alternate source of power are called self-powered devices. A bus-powered device has its positive supply voltage connected as soon as it is plugged into the bus. In a self-powered device, the supply voltage may be off when the device is plugged into the USB bus. Then the power may be turned on. In both of these cases, the 1.5Kxcexa9 pull-up resistor immediately starts pulling up one of the data lines as soon as power is applied. Therefore, these devices must have completed power-up reset procedures prior to the host polling the device for information. It is possible that these devices may not have completed their power-up reset procedures and that being polled by the host will cause errors or an unstable device state. Thus, a solution that can at least provide more time for power-up reset procedures is desirable.
A preferred embodiment of the invention allows a USB device signal connection and disconnection from the bus without physically removing or reinserting the USB cable. The devices system controller controls the signaling so that connection or disconnection may be signaled in a variety of situations to accomplish certain tasks. The invention eliminates the need for a USB device to be ready prior to the insertion of the USB cable. It also eliminates the need for a USB device to complete its power-up reset procedures prior to being polled by the host. The invention also provides a USB device with the ability to reinstate normal communications by signaling a disconnect and then a reconnect so that the host device will reinitialize communications and setup of the USB device. The invention also allows the same data lines to be re-used when operating the device with a different type of interface.
The invention provides a switched connection from a USB data line through a pull-up resistor to a positive supply voltage. The switched connection is controlled by a logic function so that it will not be connected when the power supply on the USB cable is not present. The switched connection is also controlled by a logic signal that may be supplied by the USB device""s system controller.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.